Do you mean a last will and testament or a living will as in health care decisions. I recently wrote an article on the distinction between those which can be found here:
Either one is best prepared under the direction of an attorney and neither need to be filed with the court, but may be if you desire it.
This answer does not constitute legal advice and no attorney client relationship has been formed. Before choosing a course of action, it is always advisable to seek the advice of an attorney in your area.
Any legal document should be prepared by an attorney. If you are modest means and cannot find counsel on AVVO call your local bar association as frequently they have lists of attorney's who will prepare these documents for reduced fees.
A living will or health care proxy is a document that states what should happen medically if you cannot communicate with a doctor your medical wishes. A will deals with disposition of material property such as a house, money in the bank, furniture.
So long as you're sure you're aware what exactly a living will is (Attorney Aminov's link is spot on for this), it can be done by yourself. Estate planning counsel may have a greater ability to make sure your wishes are properly written and honored, and ensure no ambiguity exists.
A living will does not have the formality requirements that a last will and testament has in New York. With the last will, the execution ceremony is more important than the document itself, and no lay person should ever undertake to do their own. (Nor should attorneys who don't focus in estate planning, either, really. Having a system in place protects the will from probate challenges, and having defended the challenges shows estate and probate counsel what they need to be sure of having just right in the wills they write.)
I am licensed in New York only. My answers are generally based on New York law or common law, except where otherwise indicated, and other states may make exceptions or have peculiarities resulting in different answers from an attorney licensed in the other state.
You need a lawyer. It is not clear from your summary what it is that you are speaking of. My guess is that a living trust is probably what you were trying to ask about. That is generally the best way to provide for a beneficiary who is a minor. As Attorney Bernick stated, any time you are contemplating signing legal documents, it is best to have a lawyer to prepare the paperwork. This is especially true when you are speaking about an estate plan that is going to take care of your child.
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A living will is basically a document that states wishes regarding healthcare decisions. If you are incapacitated or unable to make the medical decisions for yourself, such as whether or not you want artificial means implemented to keep you alive, your living will provides those directives. I agree with the others who suggest you speak with a lawyer near you. Whether it is a Living Will or Last Will and Testament that you are seeking, an attorney can guide you through the process to be sure the document meets your needs. Since you said you have a son, this type of planning is particularly important. Many attorneys in this area offer free consultations; you can search the New York State Bar Association website for attorneys near you.